1. Field of the Invention
The present invention relates to a direct-view display apparatus, and more particularly to a direct-view display apparatus that is suitable for use as display means such as a viewfinder for a video camera or a head-mounted display for a virtual reality system.
2. Description of the Related Art
The display panel used in the invention is an optically transmissive display panel that does not emit light by itself but that displays images and characters by modulating the intensity of the light emitted from a separately provided light source, the transmissivity of the panel being varied by the application of a drive signal. Examples of this type of display include liquid-crystal display panels, electrochromic displays, and displays utilizing light-transmitting ceramics such as PLZT; among others, liquid-crystal display panels are extensively used in portable televisions, word processors, projectors, etc.
To realize full color display on portable televisions, word processors or the like with such a liquid-crystal display panel, red, green, and blue color filters are provided, each aligned with a pixel in the display panel. In the case of a viewfinder for a video camera and a head-mounted display for a virtual reality system also, similar color filters are provided to produce full color display.
The liquid-crystal panel contains pixel electrodes that are arranged orderly in a matrix pattern. An independent driving voltage is applied to each individual pixel electrode, thereby changing the optical properties of the crystal and thus producing images and characters on the display. There are two basic methods of applying an independent driving voltage to each pixel electrode: one is the direct-matrix system, and the other is the active-matrix system that utilizes nonlinear two-terminal devices or three-terminal devices. Specifically, in the latter system, since it is necessary to provide switching devices, such as MIM (metal-insulator-metal) devices or TFT (thin-film transistor) devices, plus bus electrodes for transferring the driving voltage to the pixel electrodes, the aperture ratio, expressed as the ratio of the effective pixel aperture (light passing area) 52 to the total area of the pixel 51, is reduced as shown in FIG. 11. Furthermore, since the normal driving voltage is not applied to the liquid crystal in areas where the switching devices and bus electrodes are formed, the liquid crystal in those areas does not function to produce the intended display. Therefore, a light-blocking means called a black matrix is provided over the areas indicated by oblique hatching in FIG. 11, so that the light passing through these areas is blocked by the light-blocking means. In such a liquid-crystal display panel, the light-blocking areas covered with the black matrix are observed as a stripe pattern.
When employing the above-described active-matrix driving system in a direct-view display apparatus such as a viewfinder for a video camera or a head-mounted display for a virtual reality system, it is required to form pixel electrodes, switching devices, and bus electrodes at high density within a restricted area. However, it is difficult to form the switching devices and bus electrodes smaller than a certain size because of constraints in terms of their electrical performance and manufacturing technology. This causes the problem that as the pixel pitch is made smaller, the aperture ratio is reduced and the display becomes darker, making the light-blocking stripe pattern due to the black matrix more noticeable and thus resulting in significant degradation of the picture quality.
To solve the problem caused by the reduction of the aperture ratio, there is proposed an approach that involves placing a microlens on each pixel of the display panel to magnify the light exiting each pixel electrode up to the size of the pixel. For example, Japanese Unexamined Patent Publication (KOKAI) No. JP-A 64-35415 (1989) discloses an example in which a light valve used in a projector or the like is realized by using a telecentric optical system, in combination with meniscus microlenses, for converting a beam of parallel light into a magnified beam of parallel light. On the other hand, there is disclosed, in Japanese Examined Patent Publication (KOKOKU) No. JP-B2 63-62748 (1988), an example in which the above approach is taken in the construction of a segment-type direct-view display.
As described above, for full color display, red, green, and blue color filters are provided, each aligned with a pixel in the display panel, and a full-color picture is displayed by various combinations of the three differently colored pixels. However, in the case of a direct-view display such as a viewfinder and a head-mounted display, the display panel is viewed from such a short distance that the size of each of the pixels is larger than the size recognized by the resolution of the observer's eye. This results in reduced smoothness of the produced image, and hence, degradation in image quality. Among others, in the case of a head-mounted display for a virtual reality system, the image quality degradation is particularly pronounced since the observer views a magnified image. This causes a serious problem when creating a virtual space.
Furthermore, in the direct-view display used as a viewfinder for a video camera or a head-mounted display for a virtual reality system, since the eyepiece is placed near the display panel, parallax in horizontal and vertical directions on the display panel becomes greater. Therefore, if microlenses such as described above are used, an optical displacement will occur between the microlens pitch and the pixel pitch of the display panel, which causes the emergent light from the display panel to pass off the center of each microlens and, as a result, produces a stripe pattern, generally known as a moire pattern, degrading image quality significantly.
Considering this problem, the aforementioned Japanese Unexamined Patent Publication (KOKAI) No. JP-A 64-35415 (1989) employs a telecentric optical system and uses collimated light to illuminate the display panel. However, for applications such as a viewfinder and a head-mounted display, compactness is also an important consideration, but it is difficult to produce rays of highly collimated light having uniform luminous distribution within a relatively restricted space. This makes is imperative to use a diffused light source such as a fluorescent lamp, which however causes image quality degradation due to moire patterns, as described above, and the effect such as described in the above Publication cannot be expected. On the other hand, the display disclosed in the aforementioned Japanese Examined Patent Publication (KOKOKU) No. JP-B2 63-62748 (1988) is not specifically designed for applications requiring placing the eyepiece near the display panel; therefore, when the display is used as a viewfinder or a head-mounted display, image quality degradation will occur due to moire patterns as described above.